Abstract
Ten percent of adults in the United States have a diagnosis of diabetes and up to a third of these individuals will develop a diabetic foot ulcer (DFU) in their lifetime. Of those who develop a DFU, a fifth will ultimately require amputation with a mortality rate of up to 70% within five years. The human suffering, economic burden, and disproportionate impact of diabetes on communities of color has led to increasing interest in the use of computer vision (CV) and machine learning (ML) techniques to aid the detection, characterization, monitoring, and even prediction of DFUs. Remote monitoring and automated classification are expected to revolutionize wound care by allowing patients to self-monitor their wound pathology, assist in the remote triaging of patients by clinicians, and allow for more immediate interventions when necessary. This scoping review provides an overview of applicable CV and ML techniques. This includes automated CV methods developed for remote assessment of wound photographs, as well as predictive ML algorithms that leverage heterogeneous data streams. We discuss the benefits of such applications and the role they may play in diabetic foot care moving forward. We highlight both the need for, and possibilities of, computational sensing systems to improve diabetic foot care and bring greater knowledge to patients in need.
Keywords
Introduction
Ten percent of adults in the United States have a diagnosis of diabetes, 1 the leading cause of health care’s rising costs in the United States. 2 Over a third of those with diabetes will develop a diabetic foot ulcer (DFU) over their lifetime, frequently leading to infections and amputations with a mortality rate of up to 70% within 5 years. 3 Communities of color are disproportionately impacted as they are more likely to develop this devastating complication of diabetes.4,5
There is increasing interest in the use of computer vision (CV) and machine learning (ML) techniques to aid the detection, characterization, monitoring, and even prediction of DFUs. Diabetes-related limb loss pathophysiology is complex and highly variable, resulting in several challenges to successful diabetic foot care. 3 Computer vision and ML may aid decision-making processes during the point of care as well as improve the continuity of care and patient engagement. This scoping review provides an overview of the benefits of interventions that incorporate CV and ML and the role they may play in diabetic foot care moving forward. We then provide an overview of relevant techniques for analysis of the at-risk foot, gait analysis, and predictive algorithms that leverage such heterogeneous data streams. This scoping review intends to bring attention to both the need for, and possibilities of, computational sensing systems which leverage CV and predictive ML algorithms to improve diabetic foot care.
Background
Diabetes and DFUs
Approximately 19% to 34% of patients with diabetes develop a DFU in their lifetime. 3 Of those who develop a DFU, 40% will have recurrent ulcers within a year after ulcer healing, 50% will develop infections, and 20% will ultimately require amputation with a mortality rate of up to 70% within five years. 3 Communities of color in the United States experience more frequent and severe diabetes complications including DFU.4-6 In 2019, for example, non-Hispanic Black individuals were 2.5 times more likely to be hospitalized with diabetes complications, as compared to non-Hispanic white individuals. 4 The cost in human suffering also translates to economic burden, as diabetes is the leading cause of the rising cost of health care in the United States. 2 Foot complications constitute a major source of the nation’s estimated annual diabetes expenditure, which totals $273 billion in direct costs and $90 billion in indirect costs. 7
Diabetes-related limb loss pathophysiology is complex with variable clinical presentation. Due to frequent foot ulcer recurrence, management and treatment benefit from early and continuous assessment by foot experts such as podiatrists. Successful management of DFUs requires close coordination between health care team and patient. Like infection, critical limb ischemia is an important indicator for both DFU occurrence and recurrence, with high risks of amputation or death. 8 Some have proposed the Comprehensive Foot Exam, also known as the Full Foot Exam, as a standard assessment for DFU management. 9 The Full Foot Exam provides a quick assessment of a patient’s risk of foot ulceration by evaluating four key domains: dermatologic, musculoskeletal, neurological, and vascular.9-11 In current practice, patients with DFUs are typically seen in the clinic in one- to four-week intervals to assess and treat their wound(s). It is therefore essential that the patients themselves are able to monitor their foot health in between visits, including changes in severity or recurrence of foot ulcers and presence of preulcerative lesions.
CV and ML
Machine learning is a subfield of artificial intelligence which aims to develop methods that identify patterns within data to “learn,” that is, to iteratively improve performance for some task. 12 Computer vision applications of ML models may train computers to identify, interpret, and/or extract information from digital images often in ways inspired by a human’s biological vision system. Applications of CV and ML suggest a range of promising solutions that could be leveraged to benefit the field of diabetic foot care, as well as wound care more generally.
The costs associated with DFUs, both economic and that of human suffering, highlight the need for scalable diabetic limb-preservation programs
4
which emphasize multidisciplinary approaches focused on screening, remission, and engaging patients in self-monitoring. While there are disparate interventions and digital health technologies to track DFUs,13,14 diabetic foot care may benefit from the development of a ubiquitous computational approach to foot health which includes monitoring before, during, and after wound
Care in Place
Studies of the interplay between the processes of place and care have evolved the concept of “Care in Place,” a topic increasingly discussed in health care. 19 Care in Place refers to the aim of reducing health disparities by providing hospital-level medical services and care to a patient within their own home. This attempt to decentralize medical care may involve a variety of interventions including wellness programs, telemedicine visits, home monitoring, mobile care service, and Smart home/Internet of Things. 13
This shift toward Care in Place has implications for a variety of conditions and patient populations. Case studies or quasi experiments have especially focused on those patients for which transportation poses a great challenge, such as older patients 20 or those in palliative care, 21 demonstrating the feasibility and efficacy of delivering hospital-quality care in the comfort of patients’ own homes. Care in Place has similarly been applied to the remote management of diabetic foot disease with a focus on digital health, Smart wearables, and telehealth. 13 The “Wound Care Without Walls” (WCWW) initiative, 22 proposed shortly after the beginning of the COVID-19 pandemic, has similarly aimed to conserve hospital resources. The WCWW “Pandemic Wound Triage System” involves triaging patients and providing as much care as possible in low-risk environments, only escalating the level of care when necessary to avoid subjecting patients to increased risk of exposure to COVID-19.
Patient Empowerment & Addressing Gaps in Chronic Care
Diabetes takes a disproportionate toll on communities of color,4,5 as they experience more frequent and severe diabetes complications. These populations at greatest risk face barriers to managing their conditions, as well. Current technology for health management is often inaccessible for individuals with low literacy and low numeracy.23,24 Provider bias and assumptions regarding potential usability and effectiveness of medical technology in minorities and other disadvantaged populations may pose yet another barrier to accessing high-quality care.25,26
Thoughtful implementation of CV and ML within a mobile health system could aid decision-making processes during point of care interaction to improve overall effectiveness of clinical care. Armed with a more detailed and thorough understanding of their condition and current wound progress, patients may be better prepared to advocate for themselves during point of care interactions with their clinician(s). Especially in cases where a patient does not have the means to see the same clinician at regularly scheduled intervals, an increased understanding of their health allows patients to advocate for their own care.
Advantages of a mobile health system intervention include that it is not limited to clinician or clinic visits and that it also enables active engagement of informal caregivers and family members. While the Full Foot Exam is traditionally performed in a clinic annually (for individuals with diabetes but no history of DFUs) or in one- to four-week intervals (for individuals with DFUs), CV and ML approaches open the door for more frequent and standardized foot evaluation. Patients and their caregivers may monitor ulcers frequently and contact a clinician in the case of recurrence or change in severity. Furthermore, timeline images may be accessible via a clinician dashboard for remote monitoring of a patient’s DFU, allowing the clinical team to intervene in between routine visits if a change in care regimen is required. 27
By automating the Full Foot Exam and providing explanations and visualizations of foot health, CV and ML may act as vital computational tools to break down barriers to care and bring greater knowledge to the patients who need it most. 13
CV for Visual Assessment of DFU
Wound surface area and depth estimates, part of the dermatologic component of the Full Foot Exam, are used to measure the DFU wound-healing progress. A ruler-based method is commonly employed by clinicians, but it can be time-consuming, subjective, and therefore inaccurate.28,29 Given images of the diabetic foot, CV algorithms may automatically analyze images of the feet to segment feet and wounds (ulcers), and extract relevant characteristics, for example, measurements, shape, and location. This may also allow for precise documentation of wound size over time, aiding clinicians in the interpretation of a wound’s response to treatment. 30
Previous work 15 has developed deep learning–based automated wound area assessment methods that integrate fully automatic wound segmentation, balancing computational efficiency and accuracy. 31 Convolutional neural networks (CNNs) are a type of neural network which are designed for processing images and require a large, properly annotated data set. Such models are easily available off-the-shelf in popular programming languages such as Python (via the PyTorch framework) or may be downloaded from public repositories (such as HarDNet-DFUS 32 ). Given access to a large enough data set of annotated foot images, a CNN approach, such as Mask R-CNN, provides a simple, flexible, and general framework for object instance segmentation. 33 After the wound area is segmented, cardinal signs of inflammation (heat, redness, swelling, pain) and the classification of potential wound discharge could be leveraged for wound severity assessment. Wound attributes including images may be taken as input to predict clinical levels of infection severity. 34 Previously, CNN-based deep learning frameworks achieved F1 scores of over 80% when assessing wounds into five conditions using the eight wound attributes of the Photographic Wound Assessment Tool (PWAT): size, depth, necrotic tissue type, necrotic tissue amount, granulation tissue type, granulation tissue amount, edges, and periulcer skin viability. 16 Wound depth and granulation tissues were also employed to grade wounds into five severities. 35
Table 1 provides a brief overview of common wound segmentation or classification model outputs and the associated CV or ML techniques used. Table 2 provides more detail on recent foot wound image studies in the literature,15,16,34-42 including the participant cohort sizes and race/ethnicity, data set size (i.e., number of foot images), and CV or ML techniques used for wound segmentation or classification. Most DFU data sets that are currently publicly available were created to address binary classification of specific wound attributes, DFU detection, or DFU segmentation. 27 As noted in Table 2, very few of these studies include explicitly annotated infection or ischemia ground truth. Typically if these important indicators are included, it is only via a visual proxy such as yellow tissue41,42 or they may be used during annotation as a medical indicator of the appropriate treatment decision. 34 The computational model of Shenoy et al 40 identifies the presence or absence of nine wound characteristics, including infection; however, they do not define their diagnostic criterion for infection. Given this gap, future work should develop models that use other nonvisual clinically relevant characteristics.
Automated Wound Segmentation and Classification at a Glance.
Abbreviations: CNN, convolutional neural network; CV, computer vision; ML, machine learning; PWAT, Photographic Wound Assessment Tool; SVM, support vector machine; YOLO, You Only Look Once.
Overview of Studies Related to Automated Wound Segmentation and Classification.
Abbreviations: CNN, convolutional neural network; CV, computer vision; DFU, diabetic foot ulcer; ML, machine learning; PWAT, Photographic Wound Assessment Tool; SVM, support vector machine.
Many of the data sets used in previous deep learning implementations were collected in-clinic or under controlled conditions, and most of the studies referenced in Table 1 used data sets of primarily or entirely pale skin. An ML model is no better than its training data; biases present in the training data set can and do propagate to the model’s output. In particular, health disparities are propagated by the current, often-biased state of databases. 43 The equity of the deployment of these methods is threatened by algorithmic bias as well as a lack of transparency due to the difficulty of human interpretability. Further data collection of dark skin wounds is imperative to ensure future technologies will truly be equitable and address long-standing gaps in chronic care. Future tools must be implemented and evaluated in diverse populations, with a focus on those most heavily impacted by diabetes and DFUs.
ML Applications for Diabetic Foot
We now provide an overview of existing research in ML for the musculoskeletal domain of the Full Foot Exam, as well as predictive algorithms for diabetic foot care. We discuss not only the integration of multivariate information from heterogeneous data sources into a unified assessment framework but also computational modeling to automatically recognize early indicators of negative disease progression. Such predictive algorithms may be the key to breaking down barriers in care and bringing greater knowledge to patients in need.
ML for Diabetic Foot Gait Assessment
Changes in several gait characteristics are commonly associated with DFUs, such as gait speed, cadence, variability, and the percentage of cycle spent in the single-support phase. 44 Perhaps the most notable change, also associated with the neurological component of the Full Foot Exam, is the development of peripheral neuropathy.44,45 When these peripheral nerves are damaged, patients with neuropathy may experience impaired sensation in the foot. Losing “the gift of pain” frequently delays clinical care of developing wounds, causing these patients to present with more advanced symptoms. Diabetes-related foot neuropathies lead to musculoskeletal foot deformities, biomechanical imbalances, foot ulcers, and ultimately delayed gait changes.46-48 Automated gait analysis may be used to track mechanical changes or conservative gait strategy due to pain, calluses, swelling, or peripheral neuropathy. Increased risk of injury may be identified even before any visible changes are present, enabling proactive intervention and potential changes to care regimens.
The technological key to automated gait monitoring is advanced signal processing methods and time-series analysis methods for change point detection. Previous systems used pressure sensor flooring, 49 or inertial measurement units (IMUs) attached to the lower back, lumber levels, tibias, or feet.45,50-52 Some previous work has relied solely on IMU data (triaxial accelerometer and gyroscope data) from a common smartphone for gait assessment. This work frequently requires standardized positioning of the smartphone’s internal IMU by holstering the phone in a belt pouch 53 or a trouser pocket.54,55
Many existing gait databases, such as the OU-MVLP database, 56 are not specific to those with gait disorders. In addition, there is high comorbidity between DFUs, obesity, and aging. Machine learning gait assessment must be able to discriminate characteristics that are indicative of progressing DFUs from those of progressing age or complications with weight. Existing work has investigated the impacts of obesity and aging on gait stability and variability.57-59
Predictive ML Algorithms for Diabetic Foot Care
Given a heterogeneous data set from patients with DFU including data streams from the components of the Full Foot Exam, an ML model may be robustly trained and evaluated through leave-one-out cross-validation protocols. Previous work showed high predictive ability for non-amputation, minor amputation, or major amputation in hospitalized patients with DFUs by leveraging five-fold cross-validation based on a variety of patient data including the presence of ischemia and/or infection. 17 Similarly, an artificial neural network provided accuracy of 82% when predicting the healing outcomes of patients with DFU based on six clinical features and supported the creation of a simple wound healing prediction software tool. 18 The heterogeneous data may also be used to train generalizable assessment methods based on multivariate sequence modeling approaches, which attempt to make inferences about temporal sequences of data points. For example, long short-term memory (LSTM) 60 is a neural network architecture which includes a memory mechanism, allowing it to model long-term dependencies. These approaches may be combined with explicit representation models, such as variational auto-encoders 61 to extract a useful and representative set of features for each datapoint. In addition, any of these assessment methods may be pretrained on aggregate patient data and fine-tuned (adapted) toward the idiosyncrasies of individual patients.
In addition to the overall assessment of disease progression, computational modeling may provide an opportunity to automatically recognize early indicators of negative disease progression via sequence forecasting (see Shi and Yeung 62 for a survey), based on previous work on attention-modeling in deep learning-based time-series analysis. 63 Long short-term memory sequential models of sensor data streams and attention weight distributions may be trained not only for each sensing modality but also for each timestep in a sequence of measurements. 64 Through the automated analysis of measurements (both modality and timestep) that primarily contribute to the overall assessment, models may be derived to predict the outcome of the DFU assessment sooner than four weeks after the last medical visit.
These approaches integrate multivariate information from heterogeneous data sources into a unified assessment framework. New data aggregation models will be required that cope with multimodality, sparsity, and potential ambiguity of the input sequences. Embedding methods 65 learn combined feature spaces directly from input data sources and cluster semantically coherent portions of the input data. Feature embeddings represent the state-of-the-art in domains such as natural language processing. Preliminary results have been reported for more signal-oriented domains, eg, acoustic signal embeddings. 66 Multivariate embeddings address sparsity issues and allow for the extraction of essential trajectories of DFU disease progression that may then be used for forecasting and early prediction. Semantic embedding facilitates effective data imputation. Attention modeling facilitates effectiveness analysis of the overall procedure, eg, by eliminating data points that do not contribute to the overall assessment result.
Limitations and Future Work
Given the complexity of foot ulceration and frequent recurrence, diabetic foot care may benefit greatly from the development of further CV and ML approaches to detect wounds and monitor their progression. Future work might consider the role of predictive ML applications that incorporate additional patient medical data such as A1c or X-rays of the foot.
The use of CV is limited by multiple factors. Computer vision models require large-scale data annotation and ground truth verification by clinical staff, which can be labor-intensive with prohibitive cost. In addition, CV is primarily based on images and is therefore limited to what can be inferred visually. More sophisticated data capturing devices such as a depth camera or thermal camera may be able to assess other clinically relevant characteristics such as depth, or infection and ischemia (without relying on a proxy indicator such as tissue color), but they are expensive and would require novel ML techniques. Another promising venue is to use video to investigate CV techniques for the capillary refill test (CRT) within the vascular domain of the Full Foot Exam. In practice, the recorded output of the CRT is binary, less than three seconds or greater than three seconds, and is not a reliable predictor of ischemia, which is an important indicator for both DFU occurrence and recurrence. 67 The incorporation of CV for automated analysis of a CRT video may allow for higher precision results even if invisible to the naked eye.
Individuals are best understood when viewed as the center of an ecological system, 68 taking into account the intersectional and multifaceted challenges of DFU management for all stakeholders: patient, caregiver(s), family, community, society, and environment. This ecological approach steers us away from a more traditional view of chronic care centered only around routine and relatively infrequent visits to clinicians and instead highlights the need for collaborative telecare, improvements to patient-clinician communication and patient-caregiver interaction, patient-gathered data, education, and engagement. Current technology for health management is often not user-friendly for individuals with low literacy and low numeracy.23,24 Design of DFU technologies must consider personal, structural, and environmental factors4,68 due to the challenges faced during real-world deployments, as explored in previous mobile health technology studies for diabetic foot care.69-71 In many cases, individuals with diabetes learn about proper footcare only after developing complications, instead of earlier in the course of their disease. 72 Thus, future work should take an ecological approach regarding three key elements that underpin efforts to prevent foot ulcers and amputations: identifying the at-risk foot; regularly promoting inspection and examination of the at-risk foot, and supporting the engagement and education of the patient, caregiver(s), family, community, and care professionals in language and methods appropriate for their needs and abilities.
Computer vision and ML offer a unique opportunity to bolster patient empowerment by providing more concrete measures and clear visualizations of wound health, as well as interpretable analyses of a patient’s health data over time. Remote monitoring and automated classification are expected to revolutionize remote wound care by allowing patients to self-monitor their wound pathology, assist in the remote triaging of patients by clinicians, and allow for more immediate interventions when necessary. 73 However, there does not currently exist a computational model that analyzes the heterogeneous biomedical data involved in the Full Foot Exam, nor does there exist an associated interface that connects patients and clinicians and assists in interpreting this data. We contribute this overview of current CV and ML literature and its applicability to diabetic foot care to provide a basis for future work in these areas.
Footnotes
Abbreviations
CNN, convolutional neural network; CRT, capillary refill test; CV, computer vision; DFU, diabetic foot ulcer; IMU, inertial measurement unit; LSTM, long short-term memory; ML, machine learning; PWAT, Photographic Wound Assessment Tool; SVM, support vector machine; WCWW, “Wound Care Without Walls”; YOLO, “You Only Look Once” object detection.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the NIH Exploratory/Developmental Research Grant Award R21 (grant no. 1R21MD017943-01 and 5K23DK124647-03) and the GT-Emory AI. Humanity seed grant.
